The invention relates to a method for signal transmission in a radio communications system. The invention is particularly suitable for use in a mobile radio or wire-free subscriber access system.
In radio communications systems, for example the second-generation European mobile radio system GSM (Global System for Mobile Communications), information (for example speech, picture information or other data) is transmitted by electromagnetic waves via a radio interface. The radio interface relates to a connection between a base station and subscriber stations, in which case the subscriber stations may be mobile stations or fixed-position radio stations. The electromagnetic waves are in this case emitted at carrier frequencies which are in a frequency band that is intended for the respective system. Frequencies in the frequency band around 2000 MHz have been provided for future radio communications systems, for example the UMTS (Universal Mobile Telecommunication System) or other third-generation systems. Two modes are envisaged for the third generation of mobile radios, with one mode being referred to as the FDD mode (Frequency Division Duplex), and the other mode being referred to as the TDD mode (Time Division Duplex). These modes are each used in different frequency bands. Both modes support a so-called CDMA (Code Division Multiple Access) subscriber separation method.
One important factor in the development of mobile radio systems is to ensure that each new version of a protocol is compatible with older versions. This ensures in particular that more recent subscriber terminals can be correctly identified and addressed in an environment of base stations which support an older protocol version and, conversely, the subscriber terminals which support an older protocol version still operate correctly in new network environments. However, this is not intended to prevent the existing messages at the control level between a subscriber terminal and base station from being expanded in order to support new services or features. For this purpose, all the signaling messages must provide a type of expansion mechanism in order that the same message may in more recent versions of the protocol include new elements, without this adversely affecting the functionality of “old” receivers.
In the case of these new expansions, a distinction is drawn in the described UMTS mobile radio system, by way of example, between critical and non-critical expansions. If the receiver of a message identifies a new element which it cannot decode (since, for example, it is defined in a newer version which the receiver does not yet know) and this element is identified as being critical, it should reject the message as being incomprehensible. The transmitter uses this rejection to deduce that the receiver supports an older version of the protocol, and the message is repeated without the additional elements of the new version. If the receiver sees an unknown element which is identified as being non-critical in the message, then it should ignore this element and attempt to decode the rest of the message correctly. When developing expansion mechanisms, it is important to minimize the necessary message overhead, and to ensure that it is possible to switch to the correct protocol version (the latest which both ends can understand) with the minimum delay.
Solutions with respect to expansions of communications protocols have been proposed for two problems:
Problem 1:
In the case of the communications protocols (for example in the radio resource part) which are known from the related art for the GSM and UMTS mobile radio system, expansion mechanisms are implemented by reserving space at the end of a message, which can be filled with expansions and which may, for example, also have a so-called flag (indicator) (for example for UMTS) that the expansion has been identified as being critical or non-critical. This has the disadvantage that the messages become larger with each new version, since there are no mechanisms to shorten the message, that is to say elements from older protocol versions cannot be removed. The current expansion is thus always at the end of the previous version.
Problem 2:
If a subscriber terminal receives a message with a critical expansion (for example in the case of UMTS) which it does not understand, then the entire message is rejected and the network makes another attempt, using the same message but with an older version of the protocol. When selecting the older version of the message, the network cannot, according to the related art, make use of any information whatsoever about protocol versions which the subscriber terminal does understand. The simplest solution for a network would thus be to use the oldest version of the protocol, for which the network can assume that every subscriber terminal supports it. However, this means that the latest version, which the subscriber terminal could support, would never be used. A further option would be to in each case always send the next older version of the message to the subscriber terminal. If necessary, this procedure might have to be repeated more than once, for example if there is a large version difference between the transmitter and the receiver, and this can lead to a considerable delay in the protocol sequence.